The use of ion implantation methods for the modification of a material's surface are known. Generally, ion implantation techniques involve forming a beam of ions and then accelerating the ions to a high energy before they are directed into the surface of a solid target. The conventional ion implantation methods are "line-of-sight", meaning that only those regions of the target directly in line with the focused beam receive the ion bombardment. This technique requires that the ions impinge the target at a plane which is as close to perpendicular as possible, relative to the focused beam, so as to reduce the amount of back-sputtering at the target. Back-sputtering of the ionic dosage is undesirable in that it correspondingly decreases the amount of retained ion dose within the target.
Therefore in order to ion implant a complex three-dimensional component, the component must be manipulated within the implantation chamber so as to expose all desired surfaces to the focused beam. In addition, any convex surfaces must be masked so as to prevent this detrimental sputtering effect. These additional procedures required when working with complex three-dimensional components, have generally prohibited the widespread, high volume use of ion implantation methods for surface modification of these types of components.
U.S. Pat. No. 4,764,394 to Conrad, entitled "Method and Apparatus for Plasma Source Ion Implantation", Aug. 16, 1988, teaches an improved method for ion implantation, particularly suited for use with three dimensional targets. Instead of the use of a focused ion beam which impinges the target on a "line-of-sight" basis, Conrad forms an ionized plasma which essentially envelopes the target, thereby making possible the ion implantation of complex, three-dimensional shaped components without the previous requirement for manipulation or masking of the component. With the method of Conrad, high volume, ion implantation of complex components appears to be possible.
To date, ion implantation methods have been useful for improving the friction, wear and corrosion resistance properties of many materials. Specifically, ion implantation methods have been a useful means for enhancing the surface wear resistance of tool steels and other alloys employed in wear resistant applications, as well as ceramics and plastics. However, aluminum alloys have been noticeably absent from the list of materials successfully treated by ion implantation. Aluminum alloys have been conventionally treated with various surface coatings such as stainless steel for enhanced wear resistance, which can be unduly expensive. Yet, an ion implanted, surface-enhanced aluminum alloy would be a desirable alternative in many applications because of aluminum's high strength-to-weight ratio. A surface-treated wear-resistant aluminum alloy would be a potential candidate for many applications where more expensive and/or heavier, wear resistant parts are currently being used, including in many automotive applications.
Applicants have determined that a preferred ionic species for implantation within an aluminum alloy might probably be that of nitrogen or carbon, particularly if the aluminum alloy contained a relatively high silicon content. This would preferably result in the formation of silicon nitrides and silicon carbides at the surface of the aluminum alloy, which is the region where wear resistance is required. However, to date, attempts to implant these ionic species within an aluminum alloy have been unsuccessful, if even attempted at all. The art has generally not viewed the surface treatment of hyper-eutectic aluminum alloys by ion implantation as a viable alternative for wear resistant applications.
Previous attempts have included the ion implantation of nitrogen into an aluminum alloy having a low silicon concentration and reinforced by silicon carbide fibers. An improvement in the wear resistance of the fiber reinforced alloy was observed. However, the implantation of nitrogen into the aluminum matrix enhanced the wear resistance of the alloy only because of the formation of an aluminum nitride metal soap phase which added to the lubricity of the component, while the silicon carbide fibers which were originally present within the alloy continued to provide the hard, wear resistant phase. In particular, where the alloy did not contain the reinforcing silicon carbide fibers, but had been ion implanted with nitrogen, the alloy tended to "plough" and deform during the wear resistance test, therefore indicating that without the reinforcing original silicon carbide fibers, the wear resistance of the alloy was not enhanced by the ion implantation of nitrogen.
Also, ion implantation of nitrogen and some other species into pure aluminum has been attempted in the semiconductor fields. The resulting aluminum nitride is characterized by good thermal conductivity properties, thereby making it a useful insulator within a semiconductor. However, these teachings have not been helpful to the formation of a wear resistant aluminum alloy component.
Therefore, it would be desirable to provide an aluminum alloy having enhanced wear resistant properties, preferably due to the formation of hard wear resistant particles, such as silicon nitride or silicon carbide particles, at the surface of the material. In addition, it is also preferred that the ion implantation be accomplished by the plasma source ion implantation methods of Conrad, so that complex parts could be readily treated in a high production environment such as for automotive applications.